What is the effect of the water temperature in the performance of the diaper?

Temperature has a small but still significant effect on the performance of the super absorbent.  It also affects capacity (to absorb) under load.  For example, a typical diaper with a SAP content of 30% by weight will result in 9 to 12%  reduction in capacity if  temperature changes from 37 C to 7 C, a total variation of 30 C.   If you are comparing diapers for a bench mark study, make sure you  perform all tests at the same temperature.   Normal room temperature  (23 C) is more typically used rather than heated saline solution at 37 C.   Also make sure that you allow the diapers to be conditioned to this temperature for at least a couple of hours before you do any testing.  Typical laboratory conditions should be 50% relative humidity and 23 C.  A small change in relative humidity has little effect on diaper performance unless you are measuring WVTR (water vapor transmission rate) used to test breathable films.